Coming to the main topic, I've got Comodo installed at Home PC with Avast 6 Free. And they co-exist very well. Keeping very low memory footprint, yet protection is rock solid. These two had been my primary protection as combination for years, before I got Bitdefender, which also worked well with Comodo, until BD's latest offering as BDAV 2012 - some compatibility issues with Comodo.
Anyway, take Comodo + Avast Free is all you need, and also keep Malwarebytes / SUPERAntiSpyware as your secondary scanners. Good Luck.